I. Bowman — Physiography of the Central Andes. 383 



slope cm the margin of the great interior basin of Bolivia, 

 descending to the left in correspondence with the gradient of 

 the vallev of the La Paz river. In the background is the even- 

 crested platform, or plateau, whose remarkable topographic 

 character is only appreciated when one realizes how exceed- 

 ingly complex is the structure upon which the relatively simple 

 physiographic outlines are developed. The material of the 

 right middle distance is soft alluvium, but the material of the 

 background is metamorphosed sedimentaries, extremely hard 

 quartzites, sandstones and schists of Silurian, Devonian and 

 Carboniferous age. An extension to the right of that part of 

 the sketch which represents the alluvium would bring one to a 

 smooth-floored basin, the Titicaca-Poopo depression ; a similar 

 extension, to the right, of the hard rocks of the right back- 



Fig. 18. 



LIMIT OF ALL 



MIT OF ROCK 



LA PAZ VALLEY 



SOFT ALLUVIUM 



CITY OF LA PAZ 



Fig. 16. Southern portion of La Paz valley from north of the city of La Paz. 



ground would lead to the fault scarp which constitutes the 

 western edge of the eastern plateau. Extended to the left the 

 sketch would represent the residual masses which constitute a 

 true mountain chain, the Cordillera Peal, and the line of 

 Xevadas which continue southeast beyond the La Paz valley. 

 The elevation of the uplifted peneplain is here about 15,000 

 to 16,000 ft. It continues south and southeast at about this 

 elevation, and the photograph, fig. 17, shows its development 

 near the village of Araca, southeast of Illimani. The view 

 looks northwest and presents with diagrammatic clearness the 

 physiographic key to the whole region — a baseleveled surface 

 now uplifted to form a plateau which is surmounted by resid- 

 ual peaks and ranges. The photograph (fig. 18) represents 

 similar features along the western base of the Cordillera Peal 

 as seen from the town of Yiachi, thirteen miles west of La 

 Paz. The foreground is alluvium, the residual range is in the 

 background and in an intermediate position is the rock plat- 

 form, the now uplifted baseleyel of erosion of a former cycle. 



